Method of forming a copper containing
protective coating prior to electro-
deposition of paint

ABSTRACT

1. A METHOD FOR FORMING A PROTECTIVE COATING ON FERROUS METAL SURFACE WHICH COMPRISES CONTACING SAID SURFACE WITH AN ACIDIC ZINC PHOSPHATE COATING SOLUTION CONTAINING FROM ABOUT 3 TO 200 MILLIGRANS PER LITER OF CU++ FOR A PERIOD SUFFICIENT TO FORM A ZINC PHOSPHATE COATING CONTAINING METALLIC COPPER ON THE SURFACE AND, THEREAFTER, ELECTROPHORETICALLY DEPOSITING A PAINT ON THE THUS-COATED SURFACE.

United States Patent METHOD OF FORMING A COPPER CONTAINING PROTECTIVECOATING PRIOR TO ELECTRO- DEPOSITION OF PAINT Werner Rausch, Stierstadt,Hans Hansen, Karben, and Gerhard Mueller, Hanan, Germany, assignors toHooker Chemical Corporation, Niagara Falls, N.Y.

No Drawing. Original No. 3,467,589, dated Sept. 16, 1969, Ser. No.587,679, Oct. 19, 1966. Application for reissue Sept. 5, 1972, Ser. No.286,505

Claims priority, application Germany, Nov. 6, 1965,

P 12 87 413.4 Int. Cl. B011: 5/02; C23b 13/00 US. Cl. 204-181 8 ClaimsMatter enclosed in heavy brackets appears in the original patent butforms no part of this reissue specification; matter printed in italicsindicates the additions made by reissue.

This invention relates to an improved process for treating metalsurfaces and, more particularly, relates to an improved method forphosphatizing ferrous metal surfaces as a pretreatment for theelectrophoretic application of a paint or lacquer to the metal surface.

It has long been known to coat ferrous metal surfaces with a thin,crystalline, zine phosphate coating, before applying a protective paintor lacquer coating to the metal surface. The provision of such aphosphate coating has been found greatly to improve both the adhesion ofthe paint or lacquer film to the metal surface and the corrosionresistance of the painted metal. Typical of the solutions which havebeen used to produce these zinc phosphate coatings are aqueous acidiczinc phosphate solutions which contain one or more accelerating agents.Exemplary of the accelerating agents which have been used are oxidizingagents, such as nitrates, chlorates, nitrites, peroxides, bromates,organic nitro compounds and the like. Additionally heavy metalaccelerators, which are nobler than iron, such as nickel, copper and thelike, have also been used.

With the recent development of various water-soluble, resin based paintsand lacquers, a great deal of work has been done on the application ofsuch coating systems by electrophoresis. The electrophoretic applicationof paint and lacquer involves the phenomena of electro-osmosis andelectrolysis, as well as electrophoresis. In this method, an electriccurrent is passed through the paint or lacquer solution while thearticle to be painted is made an electrode, usually the anode, in thepaint or lacquer. When using such electrophoretic application techniqueson phosphate coated ferrous metal surfaces, the resulting paint filmshave often been found to have numerous minute depression or pin holes.Such lacquer films have, generally, provided only a slight corrosionprotective action, probably because of the much smaller paint filmthickness in the depressions. Moreover, in some instances, pin holes orpores have been found in the coating which extend to the phosphate layersubstrate.

In an attempt to overcome this problem, paint and lacquers have beenused with synthetic resin components to form films which during baking,and particularly during heating to the baking temperature, soften sothat the surface blends smoothly. This technique, however, has not beencompletely satisfactory in that with such resin films, there has oftenbeen found to be a withdrawal of the paint film from the edge of theworkpiece being treated so that these places are then subjected toadditional corrosive attacks.

It is, therefore, an object of the present invention to provide animproved method for pre-treatment of ferrous metal surfaces prior to theelectrophoretic application of a protective coating to the metalsurface.

Re. 27,896 Reissued Jan. 22, 1974 A further object of the presentinvention is to provide an improved process, including the pretreatmentof the metal surface, for electrophoretically applying a protectivecoating to ferrous metal.

These and other objects will become apparent to those skilled in the artfrom the description of the invention which follows.

Pursuant to the above objects, the present invention includes a processfor treating ferrous metal surfaces which comprises contacting the metalsurface to be treated with an aqueous acidic zinc phosphate solutioncontaining, in addition to zinc and phosphate ions, from 3 to 200milligrams per liter of Cu++, maintaining the surface in contact withsaid solution for a period sufficient to form a copper containing zincphosphate coating on the surface and, thereafter, electrophoreticallydepositing a paint on the thus-coated surface. By this method, it isfound that the surface depressions and pin holes in the paint or lacquercoating are substantially eliminated and there is obtained a coatedmetal surface which has excellent corrosion resistance.

More specifically, in the practice of the method of the presentinvention, the ferrous metal surfaces to be treated are contacted withan aqueous acidic zinc phosphate coat ing solution containing from about3 to 200 and preferably from about 15 to milligrams per liter of Cu++.Various zinc phosphate solutions, containing the copper in the aboveindicated amounts, which are useful for forming protective coatings onferrous surfaces, as are known in the art, may be used in the process ofthe present invention. Typically, the solutions are aqueous acidicsolutions containing zinc ions and phosphate ions.

Additionally, these solutions may also contain accelera tors, such asnitrates, nitrites, bromates, peroxide compounds and the like.Components to influence the layer structure and thickness may also beincluded in the composition, such as condensed phosphate, organicamines, alkaline earth metal salts, such as calcium salts, and the like.Activating components, such as simple and/or complex fluorides, such asF, BF SiF ZnF and the like may also be added as well as other modifyingions such as ferrous ions, nickel ion and the like.

Exemplary of zinc phosphate solutions which may be used are thosedescribed in US. Patents 2,835,617; 3,090,709; 3,161,549; 2,813,812;2,734,204; 3,015,594; 2,540,314; 2,514,149; 2,591,479; and 2,487,137.Although the specific concentration of the phosphate coating solutionsused depends on the type of coating desired and the specific operatingconditions utilized, in general it has been found to be desirable thatthe solutions used contain from about 0.5 to 4% by weight of phosphate(P0 and have a free phosphoric acid content within the range of about0.3 to about 0.8%. Additionally, the accelerators and modifying ions aretypically present in amounts within the range of about 0.001 to about 5%by weight of the solution, depending upon the particular ions added andthe nature of the coating which is desired.

In formulating these treating solutions, zinc phosphate, zinc oxide,phosphoric acid, and other suitable sources of zinc ions and phosphateions may be used. The various accelerating, activating, or othermodifying agents may be added in any suitable water-soluble and/ordispersable form, so long as the anions or cations added with themodifying materials are not detrimental to either the coating solutionor the coatings which are produced. The copper ions may be included inthe coating solution using copper oxide, copper carbonate or the like,supplemented with acid, or by means of suitable copper salts, such ascopper nitrate, copper sulfate or the like.

The zinc phosphate phosphatizing solution may be applied to the metalsurface to be treated in any convenient manner, such as by spraying,submerging, dipping, spreading, brushing, and the like, with sprayingtechniques frequently being preferred. The temperature of thephosphatizing baths of the time of the application to the metal surfacesmay be between room temperature and the boiling point of the solution,i.e., from about 20 degrees centigrade to 100 centigrade, withtemperatures in the lower part of the range often being preferred.Similarly, the contact times between the coating solution and thesurface to be treated will be those which are normally used in applyingzinc phosphate coatings and will vary according to the nature of thecoating desired and the solution operating conditions. Typical contacttimes are from several seconds, e.g., five seconds, up to 5 minutes ormore depending upon the type of application techniques which are used.

In many instances, it has been found to be desirable to control theapplication of the phosphate coating, by selection of the coatingsolution and condition used, so that the coating produced has a coatingweight which is not substantially in excess of about grams per squaremeter, with coating weights of from about 1 to 6 grams per square meterbeing preferred. In general, it has been found desirable that thecoatings produced contain copper, deposited on the steel surfaces duringthe coating operation, in an amount within the range of about 0.03 toabout 1.6 grams per square meter, with amounts within the range of about0.15 to 1.3 grams per square meter being preferred. In many instances,because of copper cementation, the phosphate coatings produced are foundto have a clear red color. That such coatings are effective in providinga good base for an electrophoretically applied coating is somewhatsurprising in that heretofore, when using copper as an accelerator in aphosphate bath, it was the practice to add copper to the coating bathonly in such small amounts that no visible copper cementation occurredon the metal surface. The reason for this has been that the corrosionresistance of the coating was found to be decreased as the coppercontent on the metal surface increased.

Following the application of the copper containing zinc phosphatecoating to the metal surface, the coating produced is, desirably, rinsedwith water following which it may be rinsed with an aqueous solutioncontaining hexavalent chromium, as i known in the art. The thusrinsedsurface may then be further rinsed, as for example with deionized water,and dried prior to the application of the protective paint or lacquercoating.

The thus-treated surface are then coated by the electrophoreticapplication of a suitable paint or lacquer. This electrophoreticapplication of the paint or lacquer may be carried out in various ways,as are known to those in the art. Typically, the coating solutionsutilized are dilute aqueous solutions having a olids content Within therange of about 3 to solids. The surface to be coated is preferably theanode and the voltages used are typically in the range of about 50 to100 volts (direct current). Typical current densities used and coatingtimes required are, respectively, from about 0.1 to 7 amperes per squarefoot and from about 10 seconds to three minutes. Normally, the paintsolution used is at substantially room temperature, i.e. aboutcentigrade, although higher temperatures, such as those from to degreescentigrade or even higher, may be used if desired. The coating appliedusing these technique are water-thinned resin paints or lacquers whichare typically aqueous solutions based on synthetic resins such as alkydresins, acrylic polymers, melamine resins, epoxy resins, and the like.These aqueous resin solutions generally have a pH of about 9 and thesolvent used is either water or an aqueous alcoholic mixture. It is, ofcourse, to be appreciated that these coating solutions may be either aspaint or lacquers, i.e., they may be either pigmented, or unpigmented,the

pigments used being those which are typical for paints of this type.

In order that those skilled in the art may better understand the presentinvention and the manner in which it may be practiced, the followingspecific examples are given. In these examples, unless otherwiseindicated, parts and percent are by weight and temperatures are indegrees Centigrade. It is to be appreciated, however, that theseexamples are merely exemplary of the present invention and are not to betaken as a limitation thereof.

Example 1 Cold rolled smooth steel sheets were cleaned by spraying fortwo minutes at 65 degrees centigrade with an aqueous solution containingtwo grams per liter of a cleaner having the following compositions:

Percent NaOH l0 Nl2HPO4 8 Titanium phosphate activator 1 Non-ionicwetting agent 7 Thereafter, the steel surfaces were rinsed by sprayingfor 30 seconds with cold water. The thus-rinsed surfaces were then givena phosphate coating by spraying for two minutes at 50 degrees Centigradewith an aqueous solution containing the following components in theamounts indicated:

Grams per liter Zinc 2.92

n 5-5 lIiO 2.3

a 0.4 NaNO 0.17

It was found that 10 milliliters of this coating solution required 12milliliters of 0.1 normal NaOH for neutralization to the phenolphthalene end point. Thereafter, the coated surfaces were rinsed byspraying for 30 seconds with cold water, sprayed for 30 seconds at 40centigrade with an aqueous solution containing milligrams per liter CrOrinsed with deionized water and then dried for 10 minutes in acirculating air oven at centigrade. The phosphate coatings thus-producedon the metal surface was found to have a coating weight of about 3 gramsper square meter. To one set of the thus-coated metal sheets, awater-soluble red pigmented epoxy paint, containing 10% solids waselectrophoretically applied, the paint application being carried out for170 seconds at a voltage of volts and a temperature of 23 centigrade.The paint thus-applied was then baked for 25 minutes at 175 centigrade.To a second series of the coated steel surfaces, a water-soluble blackpigmented epoxy paint, containing 9% solids was electrophoreticallyapplied. The paint application being carried out for 2 minutes at 125volts, at a temperature of 30 degrees centigrade. The paint thus-appliedwas then baked for 30 minutes at 175 centigrade. To a third series ofthe phosphate coated steel, a conventional epoxy lacquer was applied bydipping the surfaces into the lacquer solution and baking the coatedsurfaces for 15 minutes at degrees centigrade. The twoelectrophoretically applied coatings were found to have numerousdepressions and pores, while the epoxy film, applied by dipping, wasfound to be substantially free of pores. The painted sheets were thenscratched diagonally with a steel needle so that bare metal was exposedat the scratch. The sheets were then exposed, with the scratched surfaceinclined upward, to the salt spray test, ASTM B117-54T, wherein thesurfaces were continuously sprayed with a 5% aqueous sodium chloridesolution at 35 centigrade. It was found that the sheets coated with theelectrophoretically applied red pigmented epoxy paint and theelectrophoretically applied black pigmented epoxy paint requiredexposure for 60 and 36 hours, respectively, to cause a removal of paintfor a distance of 3 millimeters from each side of the scratch. The sheetdip coated with the epoxy paint, however, required exposure for 288hours to cause paint removal for 3 millimeters on each side of thescratch.

In the following examples, the procedure of Example 1 was repeated withthe exception that various additives were ade to the phosphatizing bathas indicated. In these examples, where copper was added, it was added asCu(NO '3H O, while SiF was added as Na SiF and F was added as NaF. Theadditives used and the results obtained were as follows:

Hours for removal of 3 millimeters of paint in salt spray test Electro-Electrophoretlphoretlcally cally Copper deapplied applied posited in redblue phosphate ptgpi Dip layer mented merited applied in grams epoxyepoxy epoxy Phosphate solution additive and amount sqJmeter paint paintpaint Example:

2- Cu, 3 milligrams/liter 0 03 60 48 120 3 Cu, 10 mill grams/liter 0. 1060 48 168 4- Cu, 30 milligrams/liter. 0. 30 288 216 216 5. Cu, 100milligrams/liter 0.90 218 120 210 6..- Cu, 30 milligrams/liter and NaGl,5 grams/liter 0. 40 288 216 216 7 NeCl, 5 grams/liter 0 72 24 216 8 Cu,30 milligrams/liter, SiFg, 1.08 grams/liter, F, 0.13 grams/liter 0. 30288 216 210 What is claimed is:

1. A method for forming a protective coating on a ferrous metal surfacewhich comprises contacting said surface with an acidic zinc phosphatecoating solution containing from about 3 to 200 milligrams per liter ofCu+ for a period sufficient to form a zinc phosphate coating containingmetallic copper on the surface and, thereafter, electrophoreticallydepositing a paint on the thus-coated surface.

2. The method as claimed in claim 1 wherein the zinc phosphate coatingsolution contains from about to 150 milligrams per liter of Cu++.

3. The method as claimed in claim 2 wherein the copper containing zincphosphate coating produced on the metal surface has a coating Weightwhich is not substantially in excess of about 10 grams per square meter.

4. The method as claimed in claim 3 wherein the copper containing zincphosphate coating produced on the metal surface has a coating weightwhich is not substantially in excess of about 6 grams per square meter.

5. The method as claimed in claim 3 wherein the copper containing zincphosphate coating produced on the metal surface contains copper in anamount within the range of about 0.03 to 1.6 grams per square meter.

6. The method as claimed in claim 4 wherein the voltages within therange of about 50 to 1000 volts, temperatures within the range of about20 to 40 degrees centrigrade, for a period of time from about 10 secondsto 3 minutes.

References Cited The following references, cited by the Examiner, are ofrecord in the patent file of this patent or the original patent.

UNITED STATES PATENTS HOWARD S. WILLIAMS, Primary Examiner

1. A METHOD FOR FORMING A PROTECTIVE COATING ON FERROUS METAL SURFACEWHICH COMPRISES CONTACING SAID SURFACE WITH AN ACIDIC ZINC PHOSPHATECOATING SOLUTION CONTAINING FROM ABOUT 3 TO 200 MILLIGRANS PER LITER OFCU++ FOR A PERIOD SUFFICIENT TO FORM A ZINC PHOSPHATE COATING CONTAININGMETALLIC COPPER ON THE SURFACE AND, THEREAFTER, ELECTROPHORETICALLYDEPOSITING A PAINT ON THE THUS-COATED SURFACE.